The invention relates to a method for monitoring the operating state of fuel cells, in which a fuel cell arrangement comprising at least one fuel cell is provided with a signal in the low-frequency range and the occurring signal response is measured and evaluated.
The invention likewise relates to a measuring arrangement for monitoring the operating state of a fuel cell arrangement comprising at least one fuel cell, in which an operating load is connected to the fuel cell arrangement, furthermore at least one feed amplifier is provided for feeding at least one low-frequency signal whose output is coupled to the fuel cell arrangement and with an evaluation device likewise coupled to the fuel cell arrangement for detecting a signal response to the at least one fed signal.
Methods of the subject-matter are known. For example, Applicant's EP 1 646 101 B1 discloses a method for determining critical operating states of fuel cells that makes use of the circumstance that, when an alternating current signal is fed into a fuel cell or a fuel cell stack, the harmonic component of the resulting signal response allows conclusions about the operating state of the fuel cell. In the preferred feeding of a current a voltage is measured as a signal response, whose harmonic components are measured and evaluated qualitatively/quantitatively. Further calculation methods for evaluating the resulting signal response, in particular on the basis of a distortion factor measurement, are disclosed in the Applicant's WO 2013/164415 A1.
Another method of the subject-matter is described in U.S. Pat. No. 8,906,568 B2, wherein this method uses two signals of very low frequencies, for example 1 Hz and 3 Hz, and also monitors occurring vibrations of higher order.
Starting point of the known measuring methods or devices is the fact that the current/voltage characteristic curve of a fuel cell has areas with strong nonlinearity as well as areas in which the characteristic curve is largely linear. Feeding a sinusoidal current into the fuel cell or a stack will result in a largely sinusoidal signal response with a low harmonic content and accordingly a small distortion factor, as long as the fuel cell operates in a linear region of its characteristic curve. However, if the fuel cell operates in an improper operating state, for example in a substoichiometric operation, one enters a highly nonlinear region of the current/voltage characteristic curve and the harmonic component of the voltage measured as the signal response increases sharply.
In the context of the present description, a low-frequency signal is to be understood as one whose frequency or frequency components are generally not above a frequency of 10 to 20 kHz. The term “alternating current signal” used in this description for simple notation is intended to include alternating currents and alternating voltages in the same manner.
Although the known method based on the measurement of the harmonic components allows to make many statements about the operating state of the fuel cell, on the other hand it requires a large effort for the evaluation. This is partly due to detecting a larger number of harmonics, thus specifically a relatively large number of frequencies, e.g. to the 9th or 11th harmonic. On the other hand, the coupling of a sinusoidal current of sufficient magnitude into a fuel cell stack, which may have a voltage of the order of 500 volts, poses technical problems, especially if the frequencies of the signal to be fed, be it a voltage or a current, are relatively low, e.g. below 10 Hz as mentioned above in the prior art. Also, since the harmonic components are a crucial factor, the distortion factor of the signal or signals to be fed must be very low, which also requires a large effort.
An object of the invention is to provide a method and a device, which allow to quickly detect critical operating states in a relatively simple and cost-effective manner.
This object is achieved with a method of the aforementioned type, in which according to the invention the provided signal comprises two or more individual signals of different frequencies, the signal response is measured at selected frequencies of the intermodulation products of the provided individual signals and the measured intermodulation signals are used to assess the operating state.
A significant advantage of the invention is that, in contrast to a harmonic analysis, no high bandwidth is required in the measurement of the signal response and one can limit oneself to a very reduced number of intermodulation frequencies. One can imprint high frequency signals and measure them at low frequencies, which are also precisely known, such that analog or digital bandpass filters can be used, for example, and FFT (Fast Fourier Transform) must not necessarily be applied. Also, the measurement can be done in real time.
For a more accurate determination of the dimension or extent of nonlinearities, it is particularly expedient if successive measurements with different amplitudes of the provided individual signals are performed and the dimension of the nonlinearity of the current/voltage characteristic curve of the fuel cell arrangement is determined by evaluating the measured intermodulation signals.
The invention particularly provides its advantages when the signal response is measured at frequencies of the intermodulation products of second order.
In many cases in practice, it is sufficient, cost-effective and can be performed quickly, if the provided signal includes exactly two individual signals and the signal response is measured at the frequency of the low-frequency real intermodulation product of second order.
The object is also achieved with a measuring arrangement of the abovementioned type in which according to the invention two or more individual signals of different frequencies are provided to the fuel cell arrangement and the evaluation device is configured to detect the signal response at selected frequencies of the intermodulation products of the provided individual signals and to measure it according to at least its amplitude to obtain at least one output signal.
In some cases, it is easier and more cost-effective, if a power amplifier is provided for each individual signal to be fed.
If the evaluation device has a bandpass for each of the selected frequencies of the intermodulation products, a cost-effective structure is possible, whereby analogue filters can also be used.
In view of the often high voltages of the fuel cell arrangement, it is particularly expedient if the output of the at least one power amplifier is connected to the fuel cell arrangement via a coupling capacitor.